In contemporary internal combustion engine technology, it is known to deliver fuel into the cylinders of the engine by means of a fuel injector associated with each cylinder. One such type of fuel injector that permits precise metering of fuel is a so-called ‘piezoelectric injector’.
Fuel injectors are favoured over alternative fuel delivery techniques, such as carburettors, since they enable a high degree of control over the volume of fuel that is injected into an associated cylinder on each combustion cycle. Such a high degree of control is extremely important in today's world in view of the increasingly stringent environmental regulations to which engine and vehicle manufactures must adhere.
Typically, a piezoelectric injector includes a piezoelectric actuator that is operable to control an injection nozzle. One such piezoelectric fuel injector is described in European patent no. EP0995901. The injection nozzle houses an injector valve needle which is movable relative to a valve needle seat under the control of the actuator. A hydraulic amplifier is situated between the actuator and the needle such that axial movement of the actuator causes an amplified axial movement of the needle. Depending on the voltage applied to the piezoelectric actuator, the valve needle is either caused to disengage the valve seat, in which case fuel is delivered into the associated engine cylinder through a set of outlets provided in a tip of the nozzle, or is caused to engage the valve seat, in which case fuel delivery is prevented.
An injector drive voltage is applied to the actuator to control an injection event comprising one or more pilot injections, a main injection, and one or more post injections depending on an operating strategy which controls the volume and timing of fuel delivery in accordance with the engine demands. The injector drive voltage is controlled by an engine control unit.
It has been observed that, using known injector control methods, there is minimal control afforded over the rate at which pressurised fuel is delivered to the combustion space. For example, it may be desirable to slow the rate at which the injection nozzle opens since this is beneficial in terms of exhaust emissions and combustion noise at some engine conditions. However, with known control techniques, this comes at the expense of an increase in particulate emissions from the exhaust.